Many analytical devices involve the simultaneous detection of charged particles at a plurality of locations. In order to do this, an array of charged-particle detectors may be used. The minimum size of these detectors is a limiting factor in determining the minimum spacing apart at which the detectors may be positioned, and thus the minimum spacing at which particles may be detected, and this spatial limitation poses a problem which must be solved in designing the device. Another frequent requirement of multiple-detector systems is that the detectors be adjustable in their relative positions, so that charged particles may be simultaneously detected at a plurality of locations, the spacings of which locations may be varied. An example of a field in which both these requirements must be met is isotope-ratio mass spectrometry.
Isotope-ratio mass spectrometers are well known in the prior art. Typically, such an arrangement will consist of an ion source for generating a beam of ions which are characteristic of the element (or elements) in the sample to be analyzed; a mass analyzer for dispersing the ions in the beam to follow different trajectories according to their mass-to-charge ratios; and a plurality of ion detectors, each of which is positioned to detect ions of a particular mass-to-charge ratio. The mass analyzer, for example a sector magnet, effectively separates the incident ion beam into a plurality of dispersed beams which are focused at different points on the focal plane of the magnet, the points at which particular particle beams are focused on the focal plane being determined by the mass-to-charge ratios of the particles. In such a device, a plurality of particle beams may be detected simultaneously, giving a rapid and accurate measurement of the isotopic composition.
For a given mass spectrometer configuration, the spacing between the positions at which ions are detected will vary depending upon the different mass-to-charge ratios of the various isotopic beams to be measured. Typically, the distance between isotope beams to be detected is in the range of a few millimeters, so that the detectors employed must be capable of detecting ion beams only a few millimeters apart.
One type of ion detector which may be used is the continuous-dynode electron multiplier. A continuous-dynode electron multiplier is a tube of high-resistivity glass which has the property that when a charged particle strikes it, secondary electrons are emitted. The secondary electrons in their turn hit the inner wall of the tube and this process is repeated causing more and more emissions, so that at the output end of the tube a large electron signal is detected. Typically, the tube is curved and diminishes in cross-section along its length. It is possible to manufacture continuous-dynode electron multipliers which are small enough to be placed a few millimeters apart, but these have some drawbacks in that they are not as reliable as the larger models, they are more expensive to produce and they do not have a large dynamic range (i.e., a constant response over a wide range of intensities) which is very important in isotope ratio analysis where the ratios of adjacent peak heights may be greater than 1,000,000:1.
Another method for detecting ion beams which are very close together is to use a channel-plate detector. A channel plate is typically a disc of high-resistivity semiconducting glass with many tiny pores, which are the openings of tiny channels through the plate, each channel acting as a continuous dynode electron multiplier. A channel plate may have thousands of pores per square millimeter and would therefore have no difficulty in detecting beams a few millimeters apart. Channel plates do, however, have drawbacks. The lifetime of channel plates is poor as they tend to burn out after a while. Also, the existence of the pores affects the observed peak shape, which depends on the position at which the ion beam strikes the plate surface.
In order to be able to look at the isotopic composition of a plurality of different materials, another desirable feature of isotopic-ratio mass spectrometers is that the ion detectors be adjustable in their relative positions, because, as stated above, for a given mass spectrometer configuration the positions of the ion beams to be detected will vary according to the mass-to-charge ratios of the isotopes in question.
An isotope-ratio mass spectrometer incorporating many of the above features is shown in U.S. Pat. No. 4,524,275 (Cottrell et al), which is incorporated herein by reference. In this device, an ion source produces a beam of ions which are dispersed by a sector magnet and detected by a plurality of ion detectors. The magnet is shaped so that the dispersed ion beams are focused on a plane which is substantially perpendicular to the optical axis. As discussed in the patent, this arrangement addresses a number of the defects found in prior isotope-ratio mass spectrometers. Sensitivity and accuracy are increased because the arrangement of the collector slits avoids the problem of off-axis beams striking part of an up-stream detector assembly and being deflected into a down-stream detector, giving a spurious signal. Further, the fact that the detectors are arranged along a plane which is substantially perpendicular to the optical axis simplifies the mechanical linkages required to alter the positions of the detectors. However, this device suffers from the prior art problem that the minimum spacing of the detectors across the focal plane is limited by their size.
Another design of isotope-ratio mass spectrometer is shown in U.S. Pat. No. 5,220,167 (Brown et al), which uses ion-optical magnification to increase the spacing between adjacent beams of a mass spectrometer. In this device, ions are produced and initially focused in the conventional way. After initial focusing, the ion beams are refocused by a magnifying focusing assembly which is located past the focal plane. The magnifying lens magnifies the beam spacing, and the focal plane, along which is located a series of staggered detector assemblies. Each detector assembly (which comprises a conversion dynode and an electron multiplier) receives one of the ion beams through an opening in the side of the assembly, the other beams travelling past to be detected by subsequent detector assemblies. Ions which have entered a particular detector assembly strike the conversion dynode and generate secondary electrons, which are detected by the electron multiplier, which extends substantially perpendicularly to the optical axis. The magnification of the focal plane of the ion beams allows the detector assemblies to be staggered next to each other along the focal plane. However, the Brown et al device has several significant drawbacks the addition of the magnifying focusing assembly greatly increases the complexity of the system, and introduces a further source of aberrations. It also involves the provision of a 30 kV supply and associated control electronics, and necessitates a substantial increase in the distance between the mass analyzer and the detector assemblies, which means that the size of the vacuum housing in which the system is situated must also be substantially increased.
It is an object of the present invention to provide a multiple-detector system for detecting multiple beams of charged particles in an analytical device which is simple in its construction and in which particles can be detected at positions which are separated by distances smaller than the widths of the detectors.
It is another object of the present invention to provide a multiple-detector system for use in an analytical device, in which particles can be detected at positions which are separated by distances smaller than the widths of the detectors, the said multiple-detector system being simple in its construction and having charged-particle detectors which are reliable, have a long lifetime and a large dynamic range.
It is still another object of the present invention to provide a mass spectrometer having such a multiple-detector system.